The present invention relates to the video encoding standard H.263 developed by the International Telecommunication Union (ITU) for very low bit-rate multimedia telecommunication and particularly to a device for generating multiple quality level bit-rates in a H.263 video encoder.
The H.263 standard developed by the ITU (International Telecommunication Union) is a part of its H.324/H.323 recommendations for very low bit-rate multimedia telecommunication. The H.263 coding scheme which is described in xe2x80x9cVideo Coding for Very Low bit-rate Communicationxe2x80x9d, Draft ITU-Recommendation H.263, May 1996, is based on earlier schemes used in H.261 and MPEG-1/2 standards, and using a Hybrid-DPCM concept comprising a motion estimation/compensation mechanism, transform coding and quantization. Each image is divided into blocks of size 16xc3x9716 pixels (called macroblocks) and the macroblock in the current picture is predicted from the previous picture using motion estimation techniques. After the prediction, the macroblock is divided into four blocks of size 8xc3x978 pixels. The prediction error is then transformed using the Discrete Cosine Transform (DCT) and the resulted coefficients are quantized and stored in the bitstream along with the motion parameters and other side information. The H.263 standard contains several improvements compared to earlier standards which allow a substantial reduction in the bit-rate while maintaining the same image quality. These improvements make it most suitable for very low bit-rate communication (but do not exclude it from being used in high bit-rate compression as well).
The H.263 bit-stream syntax defines the structure of the coded data from the basic block data to the entire image. The quality of the reconstructed video sequence can be controlled by changing the quantization step in the encoding process according to a pre-defined rate control mechanism. This allows a flexibility in generating a video sequence according to a desired bit-rate or image quality.
When a system is designed to transmit video content to a wide range of communication channels (video servers or video-on-demand), it is highly desired to be able to use one compressed video sequence which can accommodate all needs as opposed to keeping several versions of the compressed video sequence each compressed to a different bit-rate according to the target communication channel.
Therefore, the main object of the invention is to provide a device incorporated in a video encoder for accommodating a wide range of communication channels without storing several video bitstreams.
Another object of the invention is to provide a device used in a video encoder for supplying different quality level video bitstreams by using the same stored video sequence with different quantization step sizes.
Accordingly, the device according to the invention is used in a video encoder comprising motion estimation means providing a predicted block for each predefined block based upon estimating the motion between the predefined block of the current image and the corresponding block in the previous image, transform means for transforming a prediction error resulting from the difference between the predicted block and the predefined block into the frequency domain, and quantizing means for quantizing the coefficients of the prediction error in the frequency domain and providing the quantized coefficients to a video multiplex coding unit, wherein the quantized coefficients are de-quantized and inverse transformed to give back the prediction error and add it to the predicted block whereby the result is provided to the motion estimation means in order to get a new current predicted block. Such a device which generates, from one video sequence, multiple video bitstreams of different bit-rates and corresponding to different quality levels comprises a number n of stages corresponding each to a quality level i=1 to n, each stage comprising computing means for reducing the prediction error in accordance with the quality level i and a corresponding quantized prediction error residual QDi, and summing means for obtaining a cumulative prediction error:       QD    TOTAL    =            ∑              j        =        0                    j        =        i              ⁢          QD      j      
corresponding to quality level i.
wherein QDj is the dequantized value of the quantized prediction error residual QDj.
The invention further provides a way to efficiently store up to four compressed video sequences corresponding to up to four different quality level compression of the same video sequence.
The major advantage of the invention is to save storage space or transmission bits when this data is respectively stored as a data file or sent on a transmission line because the motion vector information are only stored once for the four different compressed video sequences. In addition, only one file (for storage) or stream (for transmission) is used as opposed to the normal case where each compressed video sequence is handled independently. One other advantage of the solution is that video decoders able to process H.263 standard bitstreams but not modified according to the new storing of data of the invention, will be however able, with no change, to reconstruct the base layer of the compressed data while discarding the rest of the sub-bitstreams it cannot decode.